The present invention relates to a hygrometer functioning by means of a capacitive method, as well as to a process for producing this hygrometer.
A known means for measuring the humidity of the air consists of using a capacitor, whose dielectric absorbs a quantity of water which is a function of the relative humidity of the air and measuring the dielectric constant thereof.
For such an instrument to function correctly, at least one of the electrodes of the capacitor must be permeable to water, have a low electrical resistance and be relatively insensitive to corrosion.
In existing sensors, the compromise between these three requirements is generally brought about by the use of a vacuum-deposited ultra-thin gold coating (approx. 100 .ANG.). Although the first two conditions are generally satisfactorily fulfilled (good electrical conductivity of the gold and good permeability because the coating is very thin) such capacitors have a poor corrosion resistance. Thus, a gold thickness of 100 .ANG. can be relatively rapidly destroyed by pollutants, particularly sulphur-based pollutants (SO.sub.2, H.sub.2 SO.sub.4) and the life of the instrument is reduced.
A conventional means for solving this problem consists of protecting the gold coating by adding filters, e.g. in the form of the sheet of cellophane, However, this unfortunately has the effect of increasing the sensor response time and therefore reducing its sensitivity.